Hello Tomol,
That should be fixed any moment now, when Firefox version 65.0 is released and the WebP image format is supported - see :
https://www.reddit.com/r/firefox/comments/ajggb1/firefox_not_showing_steam_workshop_images/
And also :
https://www.ghacks.net/2018/11/02/firefox-65-supports-googles-webp-image-format/